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Sussex physicists celebrate as global experiment detects first neutrinos

Physicists from the University of Sussex working on the world’s longest-distance neutrino experiment are celebrating their first sightings of the mysterious particles.

A graphic representation of one of the first neutrino interactions captured at the NOvA far detector in northern Minnesota. The dotted red line represents the neutrino beam.A graphic representation of one of the first neutrino interactions captured at the NOvA far detector in northern Minnesota. The dotted red line represents the neutrino beam

Workers at the NOvA hall in northern Minnesota assemble the final block of the far detector in early February 2014, with the nearly completed detector in the background.Workers at the NOvA hall in northern Minnesota assemble the final block of the far detector in early February 2014, with the nearly completed detector in the background

The experiment consists of two huge particle detectors placed 500 miles apart, and its job is to explore the properties of an intense beam of ghostly particles called neutrinos. 

Neutrinos are abundant in the atmosphere, but they have barely any mass and very rarely interact with other matter. Many of the neutrinos around today are thought to have originated in the big bang - studying them could yield crucial information about the early moments of the universe. 

Dr Hartnell says: “Observing our first neutrinos is a really important milestone for Sussex, NOvA and the global physics community: we have demonstrated that our experiment is working.” 

Physicists theorise that the big bang created equal amounts of matter and antimatter. When corresponding particles of matter and antimatter meet, they annihilate one another. But somehow we’re still here, and antimatter, for the most part, has vanished. 

The best measurements to date show that matter and antimatter behave almost identically and so can’t explain how we’re all here. Scientists believe that neutrinos could hold the key to understanding this mystery. 

Different types of neutrinos have different masses, but scientists do not know how these masses compare to one another. A goal of the NOvA experiment is to determine the order of the neutrino masses, known as the mass hierarchy, which will help scientists narrow their list of possible theories about how neutrinos work. 

Scientists generate a beam of the particles for the NOvA experiment using one of the world’s largest accelerators, located at Fermi National Accelerator Laboratory (Fermilab) near Chicago. They aim this beam in the direction of the two particle detectors, one near the source at Fermilab, and the other in Ash River, Minnesota, near the Canadian border. 

The particles complete the 500-mile trip in less than three milliseconds and billions are sent every two seconds. Because neutrinos rarely interact with other matter, they travel straight through the Earth without a tunnel. 

The Sussex team has played a crucial role in calibrating and fine-tuning the detector. 

The detector produces light when charged particles pass through it. 

Sussex’s Dr Abbey Waldron and PhD student Luke Vinton have developed a calibration procedure that uses particles called muons – which are understood well by physicists - as a ‘standard candle’ to allow precise measurements of less-well-understood particles like neutrinos. 

As part of the calibration procedure, the team realised that they were missing some small flashes of light produced by the neutrinos, so they fine-tuned the detector to make it more sensitive, allowing scientists to see the neutrinos interacting throughout the huge detector. 

The detector sees 200,000 particle interactions a second, produced by cosmic rays bombarding the atmosphere, and scientists can't record every single one. Sussex’s Dr Matthew Tamsett has developed a trigger algorithm that searches for events that look like neutrinos among the billions of other particle interactions.